docs: def export(self, nidm_version, export_dir):
          """
          Create prov entities and activities.
          """
          # Create "Cluster definition criteria" entity
          if isinstance(self.connectivity, int):
              if self.connectivity == 6:
                  self.connectivity = NIDM_VOXEL6CONNECTED
              elif self.connectivity == 18:
                  self.connectivity = NIDM_VOXEL18CONNECTED
              elif self.connectivity == 26:
                  self.connectivity = NIDM_VOXEL26CONNECTED
  
          # FIXME if connectivity is missing
          if self.connectivity is not None:
              atts = (
                  (PROV['type'], self.type),
                  (PROV['label'], self.label),
                  (NIDM_HAS_CONNECTIVITY_CRITERION, self.connectivity))
          else:
              atts = (
                  (PROV['type'], NIDM_CLUSTER_DEFINITION_CRITERIA),
                  (PROV['label'], label))
  
          self.add_attributes(atts)